Need to gain 2-3 knots

Discussion in 'Props' started by Three Ts, Jul 31, 2014.

  1. JSL
    Joined: Nov 2012
    Posts: 811
    Likes: 64, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    Make sure your RPM, power, prop dia. are calculated properly. You may??? need a bigger reduction and this will change everything.
     
  2. Three Ts
    Joined: May 2011
    Posts: 37
    Likes: 2, Points: 0, Legacy Rep: 28
    Location: San Diego

    Three Ts Junior Member

    The prop models on Boatdiesel website indicate that a 1.5:1 gear with about a 16-17" square prop is close to ideal - I don't know how accurate these models are, but that is the result. By my calcs the current shaft angle is about 16 degrees relative to the hull bottom. I don't think I could or would make it much steeper. I also don't think I could get to a 20" dia prop no matter what I did, but thanks for the offer from Fastfred. First thing will be a temporary test of hull extension somehow, just to see what happens. Then, check how far I could move rudder aft within existing hull section. If not meaningful enough, then consider moving rudder into extended hull area which would solve the restriction on shaft length and prop diameter.
     
  3. Three Ts
    Joined: May 2011
    Posts: 37
    Likes: 2, Points: 0, Legacy Rep: 28
    Location: San Diego

    Three Ts Junior Member

    I took a few photos of the running gear and hull to illustrate current situation. I didn't notice the wear patterns on the rudder until seeing the photos - all the little barrnacles are gone along parts of the rudder - does this look like it could be the result of cavitation created by the prop? It's weird that it only occurs in a limited area of the rudder. You can also see that there is little deadrise, and the hull shape coming forward is still fairly flat and full. Shouldn't this make planing generally easier than with a more deep V hull shape, even if it results in more pounding?

    One more question regarding shaft angle: I've calculated that the existing shaft angle is about 16 degrees. Per one prior comment, increasing the shaft angle is one way to accommodate a larger diameter prop. Is there a maximum angle that one should not exceed?
     

    Attached Files:

    • 019.jpg
      019.jpg
      File size:
      419.6 KB
      Views:
      290
    • 020.jpg
      020.jpg
      File size:
      405 KB
      Views:
      267
    • 022.jpg
      022.jpg
      File size:
      449.4 KB
      Views:
      275
    • 023.jpg
      023.jpg
      File size:
      512.2 KB
      Views:
      284
    • 027.jpg
      027.jpg
      File size:
      499.2 KB
      Views:
      275
  4. baeckmo
    Joined: Jun 2009
    Posts: 1,654
    Likes: 670, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    With the high propeller loading comes a more intense rotation in the slipstream after the propeller. This will "clean" the rudder surface along the whirling streamlines. It is not necessarily a result of the cavitation, since most of the cavities are collapsing on, and just after the blades.

    You should not plan any increase of the shaft angle, the propeller working "environment" will get worse, and you will lose much of what you gain with a bigger prop.

    In order to accommodate an 18" propeller (about the biggest suitable for the existing gearing) requires the prop to be moved 184 mm aft; for 17", 140 mm is enough. Can the rudder stock be moved correspondingly and still stay inside the transom?

    The difference in thrust between the 1,5:1 gearing and a 2,0:1 is only about 8,5 % (noncavitating that is....) and is to some extent eaten up by the increased weight and increased resistance of the beefier shaft and bracket, so I don't think there is much to be won by changing gear ratio. Better to focus on the overweight issue and make room for a better propeller for the tranny you already have.
     
  5. Three Ts
    Joined: May 2011
    Posts: 37
    Likes: 2, Points: 0, Legacy Rep: 28
    Location: San Diego

    Three Ts Junior Member

    Rudder log is only 5" from stern as is, so the only way to lengthen the shaft enough is to add a hull extension. This will be quite a bit of work, but I think it is the only thing that makes sense, along with removing other weight wherever possible. Since all of this would ultimately involve new shaft and strut, would it be acceptable to replace with a 1.25" shaft instead of current 1.5"? Costs would be a little less, and the selection of available props (used) seems to be much greater for props in the 16-17" diameter range with a 1.25" shaft.
     
  6. Yobarnacle
    Joined: Nov 2011
    Posts: 1,746
    Likes: 130, Points: 63, Legacy Rep: 851
    Location: Mexico, Florida

    Yobarnacle Senior Member holding true course

    My boats have 17 inch props on 30 mm shafts (1 3/16 inch)
     
  7. baeckmo
    Joined: Jun 2009
    Posts: 1,654
    Likes: 670, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    I do not recommend making a full hull extension; only an extension of the bottom line. The reason is that from the photo it seems she is sitting slightly nose heavy. She does not need more static buoyancy aft, just more dynamic lift in the hump range. The trim tabs seen in the pics; are they in the position you normally run them?

    It's an easy job to move the rudder log. It may be installed completely "outboard" (onto the bottom extension) or within a vertical V-shape extension to the transom, whichever is easiest for the steering equipment.

    Stay with the 1.5 in shaft diameter. Changing dia will bring more trouble with the stern tube and sealing. The bigger shaft will give a safer operation with the torque variations you have, but you could consider turning down just the propeller attachment conus, if that gives you access to more propellers.

    A question: what is the beam at the waterline?
     
  8. baeckmo
    Joined: Jun 2009
    Posts: 1,654
    Likes: 670, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Yes, but what torque, and how long between bearings?
     
  9. whitepointer23

    whitepointer23 Previous Member

    Make a stainless bolt on cav plate and mount the rudder on that. You can gain a foot easy that way. Have a look at small inboard ski boats. External rudder on a cavitation plate was a common way of getting a shallow shaft angle.
     
  10. Yobarnacle
    Joined: Nov 2011
    Posts: 1,746
    Likes: 130, Points: 63, Legacy Rep: 851
    Location: Mexico, Florida

    Yobarnacle Senior Member holding true course

    Prop mounted immediately aft of keel deadwood, no strut, and designed for a 20 to 50 hp diesel (various sister ships, different owner prefferences).
    25 ft boat x 8.5 beam x 30 inch draft, planes with the 35 hp + motors.
     
  11. baeckmo
    Joined: Jun 2009
    Posts: 1,654
    Likes: 670, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    With 50 hp you would have a shaft speed of about 1540 rpm in order to cope with a 17" propeller. That gives a maximum torque of roughly 230 Nm.

    The torque we are dealing with in the Uniflite case is about 600 Nm, ie 2,6 times your example. Any comparison is thus completely out of proportion, and shows the risk of getting it all wrong if you don't check the numbers!
     
  12. kapnD
    Joined: Jan 2003
    Posts: 1,300
    Likes: 414, Points: 83, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    Are you certain that the motor is running properly? How does no load rpm compare to top rpm in gear? Does the motor smoke a lot?
    These questions are the start point, from there we go to the propeller.
    The prop has obviously been altered, probably chopped down in diameter, who knows how accurately or professionally done. This can have extremely negative affect on performance.
    You need to get a real weight for the rig, guessing at it is ineffective.
    Take it to a good prop shop and have it scanned, they will probably credit you the cost of the scan on a new prop.
    The rounded bilges are not very effective for coaxing speed out of an underpowered hull, and the chine placement may be less than ideal, hard to tell from the photos.
    I have successfully used a dqx 4 blade prop in similar situations with good results.
     
  13. Three Ts
    Joined: May 2011
    Posts: 37
    Likes: 2, Points: 0, Legacy Rep: 28
    Location: San Diego

    Three Ts Junior Member

    Weight estimate is reasonably accurate -I had it weighed on a truck scale while on the trailer with 1/2 full tanks (fuel) and about 6 gal fresh water: 8,280 lbs. I haven't had a chance to weight the trailer separately, but my rough guess is 1,000 to 1,200 lbs. Then, figure adding back in 400-500 lbs for people weight, so somewhere between 7,500 to 8,000 lbs of total displacement depending on amount of fuel, people, bait tank full or empty, water tank more or less water.

    I agree that at rest the boat the boat floats slight bow heavy, so I understand about not adding ballast at the stern to float the aft end higher.

    Here is a picture I found on the internet of a cavitation plate with a rudder mounted in it - it this generally what is being suggested? This would definitely be the easiest solution for me to do as a DIY project versus a full sealed hull extension. Would a center cav plate like this provide enough added lift, or would it still be best to try to make cav plates full width of hull bottom, sort of like the ski boat picture? Also, with full cav plates, then the trim tabs become awkard to reinstall, and maybe not necessary if cav plates are somehow adjustable. Could a center cav plate with rudder assembly be added, and then still leave trim tabs as is on either side? Would it be wrong to have tabs operating forward of the aft end of a center cav plate, or could this still make sense? If so, should tabs be lengthened to match length of cavitation plate?

    I like the whole idea of the cav plate with outboard rudder mount mainly because I can picture being able to do this myself if I have someone fabricate the actual plate(s). Also, it almost goes without saying, but I presume that any cav plates should essentially be a straight line projection of the hull shape going aft, so with a slightly curved bottom, the plates should mimic that curve?
     

    Attached Files:

  14. baeckmo
    Joined: Jun 2009
    Posts: 1,654
    Likes: 670, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    You may exclude the small radius at the sides and the spray rail, but make the bottom extension all the way in between, following the bottom transverse curvature, and along straight longitudinal lines ("buttocks"). The slight bottom curvature makes the added pieces stronger.

    Don't bother with trim tabs at all in the first place. In post 22 I asked about the trim tab angles seen in the photo, and the beam of the boat; still curious in order to supply correct info to you!
     

  15. Three Ts
    Joined: May 2011
    Posts: 37
    Likes: 2, Points: 0, Legacy Rep: 28
    Location: San Diego

    Three Ts Junior Member

    The boat has a maximum beam of 8', but I imagine the actual beam at waterline is closer to 7' at the widest point - I probably won't have a chance to verify this for another week, but based on my memory of the vertical hull taper amidships I think that is close.

    The tab position shown in the photo is the position used while running. Strangely, even though the boat seems to float slightly bow heavy when at rest, it feels bow-high under way at 11-12 kts. If feels like it is still squatting in the water and is definitely not on plane - feels like it is just at the hump but can't get over the hump. Putting the tabs down helped gain about .5 kt over having tabs up. It also helped a little when I had my son (220 lbs) stand near the bow instead of being in the cockpit. I have a feeling that the bow would drop if the boat were fully up on plane. If so, the tabs may not be needed when if I can get the speed up to 14 kts or so, which is where I think it will start to plane. When trolling at 6-7 kts now, tabs aren't needed.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.